Liquid crystal display

Glass substrate
with ITO electrodes. The
shapes of these electrodes will determine the dark shapes that will appear when
the LCD is turned on. Vertical ridges are etched on the surface so the liquid
crystals are in line with the polarized light.

Twisted
nematic liquid crystals.

Glass
substrate with common electrode film (ITO) with horizontal ridges to line up with
the horizontal filter.

Horizontal
filter film to block/allow through light.

Reflective
surface to send light back to viewer.

A
Liquid Crystal Display,
or LCD, is a thin, flat display device made up
of any number of color or monochrome pixels arrayed in front of a light source or reflector.
It is prized by engineers because it uses very small amounts of electric power,
and is therefore suitable for use in battery-powered electronic
devices.

Each
pixel (picture element) consists of a column of liquid crystal molecules suspended
between two transparent electrodes,
and two polarizing filters, the axes
of polarity of which are perpendicular to each other. Without the liquid crystals
between them, light passing through one would be blocked by the other. The liquid
crystal twists the polarization of light entering one filter to allow it to pass
through the other.

The
molecules of the liquid crystal have electric charges on them. By applying small
electrical charges to
transparent electrodes over each pixel or subpixel, the molecules
are twisted by electrostatic forces. This changes the twist of the light passing
through the molecules, and allows varying degrees of light to pass (or not pass)
through the polarizing filters.

Before
applying an electrical charge, the liquid crystal molecules are in a relaxed state.
Charges on the molecules cause these molecules to align themselves in a helical structure, or twist (the
"crystal"). In some LCDs, the electrode may have a chemical surface that seeds
the crystal, so it crystallizes at the needed angle. Light passing through one
filter is rotated as it passes through the liquid crystal, allowing it to pass
through the second polarized filter. A small amount of light is absorbed by the
polarizing filters, but otherwise the entire assembly is transparent.

When
an electrical charge is applied to the electrodes, the molecules of the liquid
crystal align themselves parallel to the electric field, thus limiting
the rotation of entering light. If the liquid crystals are completely untwisted,
light passing through them will be polarized perpendicular to the second filter,
and thus be completely blocked. The pixel will appear unlit. By controlling the
twist of the liquid crystals in each pixel, light can be allowed to pass though
in varying amounts, correspondingly illuminating the pixel.

Many
LCDs are driven to darkness by an alternating current, which disrupts the twisting
effect, and become light or transparent when no current is applied.

To
save cost in the electronics, LCD displays are often multiplexed. In a
multiplexed display, electrodes on one side of the display are grouped and wired
together, and each group gets its own voltage source. On the other side, the electrodes
are also grouped, with each group getting a voltage sink. The groups are designed
so each pixel has a unique, unshared combination of source and sink. The electronics,
or the software driving the electronics then turns on sinks in sequence, and drives
sources for the pixels of each sink.

Brief
history

The first
operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced
in 1968 by a group at RCA headed by George
Heilmeier. Heilmeier founded Optel, which introduced a number of LCDs based
on this technology. In 1969, the twisted nematic
field effect in liquid crystals was discovered by James
Fergason at Kent State University,
and in 1971 his company (ILIXCO) produced
the first LCDs based on it, which soon superseded the poor-quality DSM types.

Transmissive
and reflective displays

LCDs
can be either transmissive or reflective, depending on the location of the light
source. A transmissive LCD is illuminated from the back by a backlight and viewed
from the opposite side (front). This type of LCD is used in applications requiring
high luminance levels such as computer displays, personal digital
assistants, and mobile phones. The illumination
device used to illuminate the LCD in such a product usually consumes much more
power than the LCD itself.

Reflective
LCDs, often found in digital watches and calculators, are illuminated by external
light reflected by a (sometimes) diffusing reflector behind the
display. This type of LCD has higher contrast than the transmissive type since
light must pass through the liquid crystal layer twice and thus is attenuated
twice. The absence of a lamp significantly reduces power consumption, allowing
for longer battery life in battery-powered devices; small reflective LCDs consume
so little power that they can rely on a photovoltaic cell, as
often found in pocket calculators.

Transflective
LCDs can work as either transmissive or reflective LCDs. They generally work reflectively
when external light levels are high, and transmissively in darker environments
via a low-power backlight.

Color
displays

In color
LCDs each pixel is divided into three cells,
or subpixels, which are colored red, green, and blue, respectively, by additional
filters. Each subpixel can be controlled independently to yield thousands or millions
of possible colors for each pixel. Older CRT monitors
employ a similar method for displaying color. Color components may be arrayed
in various pixel geometries, depending
on the monitor's usage.

Passive-matrix
and active-matrix

LCDs
with a small number of segments, such as those used in digital
watches and pocket calculators,
have a single electrical contact for each segment. An external dedicated circuit supplies an
electric charge to control each segment. This display structure is unwieldy for
more than a few display elements.

Small
monochrome displays such as those found in personal organizers, or older laptop
screens have a passive-matrix structure employing supertwist nematic (STN) or
double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with
STN). Each row or column of the display has a single electrical circuit. The pixels
are addressed one at a time by row and column addresses. This type of display
is called a passive matrix because the pixel must retain its state between refreshes
without the benefit of a steady electrical charge. As the number of pixels (and,
correspondingly, columns and rows) increases, this type of display becomes increasingly
less feasible. Very slow response
times and poor contrast are typical of passive-matrix
LCDs.

For high-resolution color displays
such as modern LCD computer monitors and
televisions, an active-matrix
structure is used. A matrix of thin-film transistors
(TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated
transistor, which allows each
column line to access one pixel. When a row line is activated, all of the column
lines are connected to a row of pixels and the correct voltage is driven onto
all of the column lines. The row line is then deactivated and the next row line
is activated. All of the row lines are activated in sequence during a refresh
operation. Active-matrix displays are much brighter and sharper than passive-matrix
displays of the same size, and generally have quicker response times.

Quality
control

Some
LCD panels have defective transistors, causing permanently
lit or unlit pixels. Unlike integrated circuits,
LCD panels with a few defective pixels are usually still usable. It is also economically
prohibitive to discard a panel with just a few bad pixels because LCD panels are
much larger than ICs. Manufacturers have different standards for determining a
maximum acceptable number of defective pixels. The following table presents the
maximum acceptable number of defective pixels for IBM's ThinkPadlaptop line.

LCD
panels are more likely to have defects than most ICs due to their larger size.
In this example, a 12" SVGA LCD has 8 defects and a 6" wafer has only 3 defects.
However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection
of the LCD panel would be a 0% yield. (The standard is much higher now due to
fierce competition between manufacturers and improved quality control. An LCD
panel with 4 defective pixels is usually considered defective and customers can
request an exchange for a new one.) The location of a defective pixel is also
important. Often manufacturers relax their requirements when defective pixels
are in the center of the viewing area.

Some
manufacturers offer a zero dead pixel policy.

Zero-power
displays

The
zenithal bistable device, developed in 2000 by ZBD Displays Limited, can
retain an image without power, but this technology is not yet mass-manufactured.

A
French company, Nemoptic, has developed another zero-power, paper-like LCD technology which
has been mass-produced in Taiwan since July 2003. This technology
is intended for use in low-power mobile applications such as e-books and wearable
computers. Zero-power LCDs are in competition with electronic paper.